Update CHANGELOG

CHANGELOG.md

@@ -12,6 +12,8 @@ FEATURES:
   that can be replaced using the command-line, files, or environmental
   variables. This dramatically improves the portability of packer templates.
   See the documentation for more information.
+* **Machine-readable output** can be enabled by passing the
+  `-machine-readable` flag to _any_ Packer command.
 * All strings in a template are now processed for variables/functions,
   so things like `{{timestamp}}` can be used everywhere. More features will
   be added in the future.